ReviewThe frontal aslant tract (FAT) and its role in speech, language and executive function
Section snippets
Anatomy and connectivity of the frontal aslant tract
Although its description in humans is relatively recent, connections between the lateral inferior frontal cortex and medial superior frontal cortex have been described previously in non-human primates. For example, Thiebaut de Schotten and colleagues (Thiebaut de Schotten et al., 2012) note the similarity of the human FAT with a fiber pathway described in a single macaque studied with autoradiography. This fiber pathway, reported in Case 25 of Schmahmann and Pandya (2006) is similar to, though
Functional associations of the left frontal aslant tract in speech and language
Given its connectivity with the IFG, which has traditionally been referred to as “Broca's area”, a region important for speech and language (Tremblay & Dick, 2016), and with the pre-SMA and SMA associated with aphasia of the supplementary motor area (Ardila & Lopez, 1984) and with speech production in typical people (Tremblay & Gracco, 2009), it is not surprising that the vast majority of studies on the function of the left FAT have focused on speech and language. Indeed, there is extensive
Functional associations of the right frontal aslant tract in executive function/inhibitory control
Although speech and language functions are distributed across several regions on both hemispheres, some aspects of speech and language are left lateralized in most right-handed individuals (Knecht et al., 2000), and the function of the left IFG has been a focus of inquiry since the time of Broca. The functional association of the right IFG has only more recently become a subject of debate. Earlier reports focused on the role of the right IFG in executive function, specifically inhibitory
Proposed function of the frontal aslant tract
As Schwan and colleagues (2012) point out, the limited physiologic data on this particular connection in humans makes it difficult to specifically determine its function at a more mechanistic level. Yet, several groups have hinted at proposed functions of the FAT, especially for its general involvement in speech. For example, Catani and Bambini (2014) proposed a broad role for the FAT in providing “a basis for intentional communicative acts”, with a potential role in social cognition. However,
Limitations and suggested areas of future research
The proposed model suggests a number of potential avenues for future research. We will focus on a few here. First, with respect to the left FAT, although we have argued for an important but perhaps not primary role for the FAT's in sequential movement planning for speech, we have grounded this on a limited empirical base investigating the tract's specific functions. Very limited research has examined different sub-components of the FAT and their associated functions. Thus, it may be the case
Conclusion
The available data suggest the existence of direct pre-SMA/SMA and IFG connectivity. We propose that this connection is a key pathway for two important functional circuits—speech and executive function/inhibitory control—that are typically examined separately but that rely on overlapping mechanisms. What this review shows is that cross-pollination of the models of these circuits may be beneficial for understanding each of them separately. In addition to understanding the basic circuitry of
Conflicts of interest
None.
Acknowledgements
This work was supported by grants from the National Institutes of Mental Health (Grant R01MH112588 and R56MH108616 to P.G. and A.S.D), and from the National Institute on Drug Abuse (U01DA041156; salary support to A.S.D.). P.T is supported by a career award from the from the “Fonds de la Recherche en Santé du Québec” (FRQS, #35016).
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